1. Field of the Invention
The present invention relates to vaporizers-condensers of air distillation equipments. It first concerns an apparatus for vaporizing oxygen and condensing nitrogen for an air distillation double column of the type comprising at least a main heat exchanger disposed in the vat portion of the low pressure column, this exchanger being of the running type and including oxygen ducts, means for causing oxygen in excess to run in these ducts, means for withdrawing total vaporized oxygen and excess liquid nitrogen through the lower ends of these same ducts, nitrogen ducts in indirect heat exchange relationship with the oxygen ducts, means for feeding nitrogen gas originating from the mean pressure column to the nitrogen ducts, and means for returning condensed nitrogen to the mean pressure column.
2. Description of Prior Art
In the equipments for the distillation of air of the double column type, the liquid oxygen which is found in the vat portion of the low pressure column is vaporized by heat exchange with nitrogen gas taken in the head portion of the mean pressure column. For a given operating pressure of the low pressure column, the temperature difference between oxygen and nitrogen which is made necessary by the structure of the heat exchanger affects the operating pressure of the mean pressure column. It is therefore desirable that this temperature difference be as low as possible, in order to minimize the expenses associated with the compression of the air to be treated which is injected in the mean pressure column.
The vaporizers-condensers of the running type are very advantageous because of their excellent heat exchange performances, and can be reliably and economically produced as a result of the technology described in EP-A-130 122 in the name of the Applicant.
However, the following problem occurs.
During a stoppage of the equipment for the distillation of air as a result of an incident (temporary cut of electricity, machine break, etc. . . ) or a programmed stoppage, the liquids accumulated on the plates of the upper column (low pressure column) and eventually in the argon mixture column which is associated with the double column, and even the liquids accumulated on the plates of the lower column (mean pressure column), if no step is taken with respect to the operation of the valve for the upward movement of enriched liquid, will be poured in the vat of the low pressure column, precisely where the vaporizer-condenser is mounted.
With units in which high purities and high yields of extraction are required, there is a large number of plates and the "working charge" of liquid thus abruptly poured in the vat of the low pressure column in the case of a stoppage, will represent a height of many meters. When the exchanger is mounted in the vat of the low pressure column and when the exit of oxygen, in the form of gas as well as liquid can only take place at the base of the exchanger, the latter, then being at least partially immersed, is not capable of repriming again when restarting the equipment.
The resumption of operation of the unit after a few moments, a few hours or even a few days of stoppage therefore requires a previous flushing of the liquid still present in the vat, while this liquid is welcome since it enables to instantaneously recharge the plates of the various columns of which it constitutes the "working charge".
To reprime the vaporizer-condenser without flushing the liquid which is accumulated in the vat, one would think either of mounting the exchanger at a sufficient height from the bottom of the vat of the column so that the liquid collected does not reach the lower part of this exchanger, or of mounting outside the column, or as an appendix or as a standing out of the column vat, an element capable of retention of this liquid. However, these solutions necessitate a large space which would not be in use in normal operation, and which would represent an excessive investment cost.